Electric furnace heater element



Patented Mar. 5, 1940 UhiITED STATES ,ELECTRIG FURNACE HEATER ELEMENT Goodwin H. Howe, Scotia, N. Y., assignor to General Electric Company, a corporation of New York No Drawing; Application December 28, 1937,

Serial No. 182,086

'1 Claims.

The present invention relates to a high resistance heater element for an electric furnace and more particularly to sintered oxidation resistance wire or ribbon. Prior to the present invention, wire consisting substantially of iron, chromium and aluminum with or without other ingredients has been made by fusion and employed as grid resistors and as heating elements in furnaces' and like devices. Alloy wire or ribbon of the above composition fabricated from fused ingredients has a big resistance. However, if the total content of aluminum and chromium in the alloy is relatively high the alloy will be very brittle and if the total content of these ingredients is low, the alloy will oxidize rapidly and will be short lived when operated at temperatures in the neighborhood of 1000" C. or higher.

It is one of the objects of the present invention to provide a high resistance wire which has a long life and is capable of withstanding oxidation in air at high temperatures. It is a further object of the invention to provide an oxidation resistant wire which contains a relatively large quantity of aluminum and chromium and which is ductile.

The novel features which arecharacteristic of my invention are set forth with particularity in the appended claims. My invention itself however will best be understood from reference to the following specification.

In the alloys which I employ, aluminum is an essential ingredient. However, the fabrication of sintered alloys containing a readily oxidizable metal, such as aluminum, is difficult owing to the ease with which aluminum oxidizes. For example, if a mixture of pulverized aluminum and other pulverized ingredients of an alloy is pressed in a mold and sintered, it will be found that even when the sintering is carried out in an atmosphere of pure dry hydrogen the resulting prod uct may be a swollen mass lacking coalescence due to the pronounced affinity of the finely divided aluminum particles for oxygen. --In my copending application, Serial No. 758,990, filed December 24, 1934, and entitled Permanent =magnet and method of making the same, of which the present application is a continuation in part, Ithave disclosed a method whereby alloys containing a readily oxidizable metal such as aluminum, may be fabricated'by a sintering process.- In the process disclosed in the prior application, I produce a foundation alloy consisting preferably of iron and aluminum in substantial-' ly equalparts by weight. Such an alloy is very brittle and may be made by firstalloying the aluminum with one or more of the other ingredients of the alloy. Since it is desirable that the foundation alloy thus produced should be capable of being pulverized or crushed easily, I preferably prepare a brittle, iron, aluminum foundation alloy containing a quantity of aluminum equal to that present in the final alloy andconsisting of about 50% by weight of aluminum and 50% by weight of iron. The foundation alloy may be formed by first heating the necessary quantity of iron in a high frequency induction or other suitable furnace to an elevated temperature, for example a temperature in the neighborhood of 1000 C. This temperature is several hundred degrees below the melting point of iron but is sufliciently high to permit alloyage of aluminum with iron. The aluminum is added in small quantities to the iron to form the alloy consisting substantially of iron and 50% aluminum and having a melting point at about 1200 C. The molten alloy is poured into a graphite or other suitable mold to solidify.

Although I prefer to employ in the manufacture of my oxidation resistant wire a foundation 2 alloy consisting of about 50% iron and 50% alumium, my invention is not limited to the use of this particular alloy. Other percentages of iron and aluminum may be employed in the foundation alloy if desired. However, although various percentages of ingredients may be employed, it will be found that, in general, the alloy consisting of about 50% iron and 50% aluminum is the most satisfactory foundation alloy to employ owing to the low temperature at which it can be manufactured and particularly the ease with which it may be crushed.

Although I prefer to employ a 50% iron and 50% aluminum foundation alloy in carrying out my process, it is possible by the use of certain pre- 40 cautionary measures to avoid the use of the foundation alloy and to make the finished product by mixing powdered iron, nickel and aluminum in the desired proportions. Such precautionary measures comprise the use of very pure ingreg5 dients particularly pure iron, the use of a very fine and dense aluminum, the proper control of gas flow and the use of hydrogen having a degree of 1 purity such as that employed in carrying out the process disclosed in my copending 59 application Serial No. 164,354, filed September 17, 1937, and entitled Method of making a sintered a1loy.

In the fabrication of a sintered wire consist ing of about 8% aluminum and 92% iron, I pref- 55 crushed until it is capable of passing through a 200 mesh screen and is then thoroughly mixed in a ball mill for about one to five hours with the remaining pulverized iron which is likewise capable of passing through a 200 mesh screen. The mixed ingredients are pressed thereafter in a rectangular mold under a pressure of about 30 tons per square inch thereby forming a bar having a substantially square cross section. I have employed a pressure of about 30 tons per square inch but higher or lower pressures may be employed it being merely essential that the pressure applied be high enough to permit the pressed material to retain its shape until sinteredp The pressed bar is sintered at a temperature above 1000" C. and generally at a temperature in the neighborhood of 1400" C. The sintering time may vary but usually is not longer than about three hours. The sintering process may be carried out by placing the pressed bars in a closed container provided with an opening through which specially pure dry hydrogen may be introduced into the container, The closed container is then placed in an ordinary hydrogen furnace heated at a temperature of about 1400 C.

It is desirable, in the sintering operation, to prevent oxidation of aluminum and this may be accomplished readily by the use of a closed tube supplied with pure dry hydrogen as hereinbefore pointed out. After the bars have been sintered, they are cooled and thereafter may be swaged and drawn into wire while heated at a temperature of 900 C. to 1100" C.

In the fabrication of sintered wires containing aluminum and chromium in addltion'to iron, for example about 8% aluminum, 8% chromium and 84% iron, the procedure employed is identical with that indicated above. In the fabrication of a wire of the above compositiona pulverized foundation alloy preferably consisting of 50% aluminum and 50% iron and containing all the aluminum present in the final alloy is mixed with the remaining pulverized iron and chromium and pressed, sintered, swaged and drawn as pointed out hereinbefore.

Wire having a diameter of .025" and produced in accordance with my process and containing about 8 to 10% aluminum with the remainder iron, is ductile and oxidation-resistant and has shown a life of more than 160 hours in air at a temperature of 1200" C. 'A similar wire containing the same ingredients but made from east or fused material when tested under the same conditions had a lifeof three hours while a well' known grade of high resistance wire of the same size containing about nickel and 20% chromium and made from east material had a life of ninety hours under the same conditions.

I also have obtained an increase in the. life of wires consisting substantially of aluminum, chromium and iron by forming the wire from mixed, pressed and sintered ingredients. For example, sintered wires .025" in diameter prepared in accordance with my process and containing 8% aluminum, 8% chromium and 84% iron had a life of about 400 hours under the high temperature oxidizing conditions noted above. A wire containing the same ingredients and made from cast material of the same composition when subjected to the same test had a life of only twentyfive hours. 1

I have made by my method, ductile, sintered oxidation-resistant wire containing iron, chromium and aluminum in which the aluminum content was about 8% and the chromium content between 4% and 37%. A sintered alloy wire containing 12% chromium and about 8% aluminum with the remainder iron, had a life of 178 hours under the above noted high temperature oxidation conditions.

I have made ductile, oxidation-resistant wire containing in addition to iron, chromium and aluminum, small percentages of other ingredients, for example 2% manganese, 2% molybdenum or 2 to 4% nickel or cobalt. Such additions however do not appear to improve the sintered iron-'chromium-aluminum alloys.

In addition-to the above compositions, I have made ductile sintered alloy wire containing 8 to 9% aluminum, 24 to 37% chromium and 55% to 67% iron, the lower iron content being associated with the higher chromium content and vice versa. Alloys containing such a high percentage of chromium and about 8% aluminum should be sintered at a higher temperature than that employed for alloys containing the same amount of aluminum and lower percentages of chromium. For example a sintering temperature of about 1450 C. to 1500 C. should be employed with the above. compositions. It will also be found desirable to pulverize the various ingredients more finely than hereinbefore indicated.

While high resistance wires containingv from about 5% to 9% aluminum and made from sintered ingredients and operated at high temperatures under oxidizing conditions resist oxidation and remain ductile after heating operations in air at temperatures as high as 1400 0., they have a far longer life than alloy wires made from a cast or fused alloy consisting of the same ingredients. The superiority of the sintered wire over the wire made from east material may be due to the uniform distribution of any impurities in the sintered alloy. In melted alloys impurities such as oxidesand brittle carbide constituents segregate at grain boundaries and are readily attackedby oxygen, In the sintered alloys however any impurities are uniformly distributed through the mass and the oxidation resistant qualities of the material are therefore far superior to the oxidation-resistant qualities of similar cast material. I

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A sintered. oxidation resistant, furnace heating element consisting principally of iron but containing an appreciable quantity of aluminum, substantially all of said aluminum having been alloyed with a portion of the iron prior to slnter- 2. A sintered, furnace heating element consisting principally of iron but containing at least 5% of aluminum, substantially all of said aluminum having been alloyed with a like quantity of iron prior to sintering, said alloy being characterized by its long life and resistance to oxidation at temperatures in the neighborhood of 1200 to 1400' c.

3. A sintered, furnace heating element consisting principally'of iron but containing about 8% to 12% aluminum, substantially all of said aluminum having been alloyed with a like quantity of iron prior to sintering.

4. A sintered, furnace heating element containing at least 57% iron, about 8% aluminum and the remainder chromium, substantially all of said 1| aluminum having been alloyed with a like quantity of iron prior to sintering.

5. A ductile, oxidation resistant, sintered, furnace heating element containing about 8% alumi- 5 num, at least 8% chromium and the remainder substantially iron, substantially all of said aluminum having been alloyed with a like quantity of iron prior to sintering.

6. A sintered, furnace heating element containing about 55% to 67% iron, about 8% aluminum, the remainder consisting of about 37% to 24% chromium, the lower iron content being associated with the higher chromium content and vice versa, substantially all of said aluminum having been alloyed with a like quantity of iron prior to sintering.

7. A sintered, furnace heating element containing about 80% to 88% iron, about 8% aluminum, the remainder consisting substantially of chromium, substantially all of said aluminum having been alloyed with a like quantity of iron prior to sintering.

GOODWIN H. HOWE. 

